Interest in genetic genealogy was expanding beyond genealogy circles by April 2006, when this week’s “Best of Family Tree Magazine” article was published. Colleen Fitzpatrick shared an example of how DNA testing can help you theorize how and where your family may have migrated.

Though not everyone’s looking to trace their roots back to the Vikings, I like this example because it shows some of the possibilities of genetic genealogy—a field where scientists continue to make door-opening discoveries for family historians.

Follow genetic fingerprints to new theories. DNA can point to a previously unrecognized episode in your family's past. “Oddball” test results sometimes signal nonpaternity events (adoptions, name changes, illegitimacies), which can link you with unexpected people and places.

Take my Fitzpatrick surname study. Although the DNA profiles (haplotypes) are relatively diverse, most of the 75 participants match one another on 20 or so markers out of 26. This shows that we share a common background—it's just far back in the past. Three people don't fit that mold, however: They match the rest of the group on no more than seven markers.

Two of these three men—a Catholic priest from New Jersey and a retired engineer from New South Wales, Australia—match each other exactly. And they've traced their families back to two small towns only 10 miles apart on the west coast of Ireland. The American's Fitzpatrick family immigrated during the Great Famine; the Australian's Fitzpatrick ancestors went “down under” in the early 1900s. How could these men match each other exactly but be so different from the rest of the Fitzpatrick study group?

Our questioning has led to some interesting theories, developed from what we know about the history of western Ireland. One potential explanation is that the men descend from a Viking who made a pit stop on his way around coastal Ireland, leaving behind a genetic souvenir. Another possibility: The pair descends from a survivor of the Spanish Armada's 1588 wreck on the west coast of Ireland.

As online databases grow to include a more diverse collection of haplotypes, we may find more matches to these men. If they match an Erikson or a Peterson, we can further probe the first possibility. If they match a Lopez or Garcia, we can explore the second theory. Or we may devise altogether new theories. But whatever we discover, they'll have a fascinating new chapter to add to their family sagas.